An asymmetrical outer retinal response to drifting sawtooth gratings

Electroretinogram (ERG) studies have demonstrated that the retinal response to temporally-modulated fast-ON and fast-OFF sawtooth flicker is asymmetrical. The response to spatiotemporal sawtooth stimuli has not yet been investigated. Perceptually, such drifting gratings or diamond plaids shaded in a sawtooth pattern appear brighter when movement produces fast-OFF relative to fast-ON luminance profiles. The neural origins of this illusion remain unclear (although a retinal basis has been suggested). Thus we presented toad eyecups with sequential epochs of sawtooth, sine-wave, and square-wave gratings drifting horizontally across the retina at temporal frequencies of 2.5 - 20Hz. All ERGs revealed a sustained DC trans-tissue potential during drift, plus a peak at drift offset. The amplitudes of both phenomena increased with temporal frequency. Consistent with the human perceptual experience of sawtooth gratings, the sustained DC potential effect was greater for fast-OFF cf fast-ON sawtooth. Modelling suggested that the dependence of temporal luminance contrast on stimulus device frame rate contributed to the temporal frequency effects, but could not explain the divergence in response amplitudes for the two sawtooth profiles. The difference between fast-ON and fast-OFF sawtooth profiles also remained following pharmacological suppression of post-receptoral activity with Tetrodotoxin (TTX), 2-Amino-4-Phosphonobutric acid (APB), and cis-Piperidine-2,3-dicarboxylic Acid (PDA). Our results indicate that the DC potential difference originates from asymmetries in the photoreceptoral response to fast-ON and fast-OFF sawtooth profiles, thus pointing to an outer retinal origin for the motion induced drifting sawtooth brightness illusion.

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